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    SubjectRe: local_add_return
    * Rusty Russell ( wrote:
    > On Saturday 20 December 2008 03:36:27 Mathieu Desnoyers wrote:
    > > * Rusty Russell ( wrote:
    > > > Then some trace-specific typedef like "trace_counter_t" which goes to local_t
    > > > or atomic_(long?)_t?
    > > >
    > > > Should be a simple patch and pretty clear.
    > >
    > > Hrm, is it me or linking a basic type definition to a single user seems
    > > like the wrong approach ?
    > Well, it's an ongoing debate. Old school kernel coders believe that
    > infrastructure should be resisted: you implement what you need to, then
    > if it turns out to be generically useful you put it somewhere that the
    > second user can access it.
    > Otherwise we end up with unused infrastructure, or overspecialized
    > infrastructure which doesn't actually meet the general need. local_t
    > displays both these properties.

    Yes.. well given every iteration on that kind of primitive touches _all_
    architectures supported by Linux, I think it's good to think a bit about
    the design in advance to minimize the amout of wasted effort. Especially
    because it requires some coordination between many architecture

    > > The idea behind declaring new types is, to me, that they should describe
    > > as generally as possible what they provide and what they are. If we
    > > think of the future, where we might want to use such local atomic types
    > > for other purposes than tracing, I think we will end up regretting such
    > > specific naming scheme.
    > I can be convinced, but I'll need more than speculation. Assuming
    > local_long_atomic_t, can you produce a patch which uses it somewhere else?

    I had this patch applying over Christoph Lameter's vm tree last
    February. It did accelerate the slub fastpath allocator by using
    cmpxchg_local rather than disabling interrupts. cmpxchg_local is not
    using the local_t type, but behaves similarly to local_cmpxchg.

    > > local_atomic_long_t is a _new_ primitive, which cannot be
    > > implemented by a trivalue and differs from atomic_long_t, on more
    > > architectures than x86. On mips and powerpc, at least, it can be
    > > implemented as an atomic operation without the memory barriers, which
    > > improves performances a lot.
    > OK, you lost me here. I don't see a memory barrier in the powerpc atomic
    > ops. Is there an implied one I missed?

    Look for LWSYNC_ON_SMP and ISYNC_ON_SMP in

    They map to the lwsync and isync instruction, which are more or less
    memory ops and instruction execution order barriers. They become both
    unneeded when modifying per-cpu data from a single CPU.

    > > I think the following scheme would be pretty simple and yet not tied to
    > > any specific user :
    > >
    > > local_long_t
    > > - Fast per-cpu counter, not necessarily atomic.
    > > Implements long trivalues, or uses local_atomic_long_t.
    > > local_atomic_long_t
    > > - Fast per-cpu atomic counter.
    > > Implements per-cpu atomic counters or uses atomic_long_t.
    > From the point of view of someone trying to decide what to use, the real
    > difference is: use local_long_t unless you need the atomic-style operators
    > which are only available on local_atomic_long_t, or NMI-safe behaviour.
    > Is this correct?


    > > We could do the same with "int" type for :
    > > local_t
    > > local_atomic_t
    > > atomic_t
    > >
    > > If we need smaller counters.
    > Erk... no, renaming one to two is bad enough. Changing the semantics of
    > one and introducing three more is horrible.
    > If we're going to rename, I'd choose local_counter_t and local_atomic_t
    > (both long: I don't think there's a real penalty is there?).

    The penality is only space and wasted cache-lines whenever the data fits
    in smaller data types, but I think we can start with a single data type
    (long) and add more if needed. I agree with you on renaming, it's bad.
    People trying to forward port their code will have a hard time managing
    the type behavior change, especially if the compiler does not complain.
    local_counter_t and local_atomic_t seems good to me, except the fact
    that atomic_t maps to "int" and local_atomic_t would map to "long",
    which might be unexpected. If possible, I'd try to follow the current
    semantics of "atomic_t" for int and "atomic_long_t" for long, because I
    think those types should offer a similar interface. I know that
    local_counter_long_t and local_atomic_long_t are painful to write, but
    that would follow the current atomic_t vs atomic_long_t semantics. Hm ?


    > Thanks,
    > Rusty.

    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68

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